In CA (Carrier Aggregation) specified up to LTE (Long Term Evolution) Release-10, as shown in Part (a) of FIG. 4, communications are performed simultaneously by using CC (Component Carrier) #1 and CC#2 under the same radio base station eNB to achieve high throughput.
Meanwhile, in LTE Release-12, “inter-eNB CA (or inter-node UP aggregation)” has been considered (see Non-patent document 1) in which CA up to LTE Release-10 is expanded such that, as shown in Part (b) of FIG. 4, communications are performed simultaneously by using CC#1 and CC#2 under different radio base stations eNB#1 and eNB#2 to achieve high throughput.
The “inter-eNB CA” needs to be performed to achieve a degree of throughput comparable to LTE Release-10 when, for example, all CCs used by one UE cannot be accommodated within one radio base station eNB.
Here, as shown in FIGS. 5 and 6, “distributed RLCs” has been proposed as a configuration to implement the “inter-eNB CA.”
As shown in FIG. 5, in the “distributed RLCs,” an M-RLC (Master-RLC) layer function of a master radio base station M-eNB is configured to generate an RLC-PDU (Radio Link Control-Protocol Data Unit) from a PDCP-PDU and forward the RLC-PDU to an S-RLC (Slave-RLC) layer function of a slave radio base station S-eNB. The S-RLC layer function of the slave radio base station S-eNB is configured to transmit the RLC-PDU to a mobile station UE.
Here, the S-RLC layer function of the slave radio base station S-eNB is configured to re-segment the received RLC-PDU when necessary.
Note that the re-segmentation is one of the RLC control processes specified in LTE Release-8, and is performed by an RLC layer function in retransmission of an RLC-PDU. Specifically, when a sufficient TBS (Transport Block Size) is not available for an RLC-PDU to be retransmitted, the RLC layer function further segments the RLC-PDU into RLC-PDU segments and transmits the RLC-PDU segments.
FIG. 6 shows a protocol stack for implementing the “distributed RLCs.”
Also, as shown in FIGS. 7 and 8, “independent RLCs” has been proposed as a configuration to implement the “inter-eNB CA.”
As shown in FIG. 7, in the “independent RLCs,” a master radio base station M-eNB is configured to transmit a PDCP (Packet Data Convergence Protocol)-PDU to a slave radio base station S-eNB. An RLC layer function of the master radio base station M-eNB and an RLC layer function of the slave radio base station S-eNB are configured to independently generate RLC-PDUs and transmit them to a mobile station UE. FIG. 8 shows a protocol stack for implementing the “independent RLCs.”